A Cowlitz County Sheriff grabs a union worker by the throat as police move in on several hundred union workers blocking a grain train in Longview, Wash., Wednesday, Sept. 7, 2011.
(AP2011)

After a union wildcat strike turned violent on Thursday, triggering a walkout at five ports in Washington state, longshoremen went back to work on Friday.

But in an unusual twist, the National Labor Relations Board succeeded in getting a federal judge to issue a preliminary injunction Thursday against further union activity at the Port of Longview after the union ignored the temporary restraining order that he issued a week earlier.

The agency will hold a hearing next month to determine whether further restrictions are warranted.

NLRB spokeswoman Nancy Cleeland said it was not common for the agency to take action against unions.

"In those cases where we do to court, the majority of petitions are against employers, but that simply reflects our cases," she told FoxNews.com. "The majority of charges filed with the NLRB are against employers."

On Thursday, hundreds of International Longshore and Warehouse Union members (ILWU) stormed a new grain terminal at the Port of Longview that they are battling for the right to work at.

They overwhelmed guards, smashed windows in the guard shack and dumped grain. Six guards were trapped for a couple of hours, Longview Police Chief Jim Duscha said. He initially referred to the guards as "hostages," but later retracted that after the guards clarified no one had threatened them.

"The guards absolutely could not get out," Duscha said. "They feared for their lives because of the size of the crowd and the hostility of the crowd."

No one was hurt, and nobody has been arrested -- although Duscha said that could change if police are able to use surveillance video or other means to identify the protesters.

A day earlier, protesters twice blocked the pathway of a train carrying grain to the terminal, leading to 19 arrests.

"Today, the ILWU took its criminal activity against EGT to an appalling level, including engaging in assault and significant property destruction," the company's chief executive, Larry Clarke, said in a written statement. "This type of violent attack at the export terminal has been condemned by a federal court, and we fully support prosecution of this criminal behavior to the fullest extent under the law."

The union believes it has the right to work at the new $200 million terminal. But labor activists insist that after receiving tax breaks and promising to create well-paying jobs, EGT initially tried to staff the terminal with nonunion workers.

Following a series of protests by the Longshore workers this year, the company announced it would hire a contractor staffed by workers from a different union the Portland-based Operating Engineers Local 701.

U.S. District Judge Ronald Leighton said there was no defense for the aggressive tactics used in recent days. He said he felt like a paper tiger because the International Longshore and Warehouse Union clearly ignored a temporary restraining order he issued last week with similar limits. He scheduled a hearing for next Thursday to determine whether the union should be held in civil contempt.

"The regard for the law is absent here," the judge said. "Somebody is going to be hurt seriously."

In Seattle, Tacoma, Everett and Anacortes, hundreds of Longshore workers failed to show up or walked off the job Thursday in apparent solidarity with the Longview activists, halting work at those ports. Union leaders said they had not called for any such actions.

"It appears the members have taken action on their own," said ILWU spokesman Craig Merrilees from union headquarters in San Francisco.

Scott Mason, president of the ILWU Local 23 in Tacoma, said some of his members have joined in the Longview effort, but he doesn't believe they were involved in illegal activity. He blamed the company for provoking the response and warned that more activity could be coming.

"How long this fight has to go on is really in their court," he said.

Paddy Crumlin, president of the International Transport Workers’ Federation, also warned of possible further action.

“EGT are playing with fire, and they know it,” he said in a statement. “They need to take a big step back and think about what they are trying to force through, then see sense and talk to the ILWI about how to resolve this issue before it escalates even further.”